Management of presentation time in a digital media presentation system with variable rate presentation capability

ABSTRACT

Method for managing Presentation Time in a digital rendering system for presentation of temporally-ordered data when the digital rendering system includes a Variable Rate Presentation capability. In a particular, one embodiment of the present invention is a method for converting Presentation Time to Data Time, and for reporting Presentation Time when only one time can be returned.

This application claims the benefit under 35 U.S.C. 119(e) of U.S.Provisional Application No. 60/255,226, filed on Dec. 12, 2000.

TECHNICAL FIELD OF THE INVENTION

One or more embodiments of the present invention pertain to the field ofcontent presentation by a digital rendering system such as, for example,and without limitation, a digital media player.

BACKGROUND OF THE INVENTION

Most traditional digital rendering systems, such as RealNetworks®RealPlayer® digital media players, maintain an internal variable duringplayback of media content that reflects a current presentation time(hereafter referred to as “Current Time”). Current Time is, in effect, acurrent “position” in the media content that is being displayed andrendered. Typically Current Time is set to zero at the beginning of themedia content, and it reaches a measure of time equal to a duration ofpresentation of the content of the entire work when the end of the mediacontent is reached.

In most traditional players, such as the RealPlayer® digital mediaplayer, a Current Time value is: (a) regularly calculated by a singlemodule; (b) acquired and stored by core routines of the player; and (c)distributed to, and utilized by, various internal player objects. Theseinternal objects utilize the Current Time value to determine when it istime to initiate or terminate various tasks associated with mediacontent playback. The calculation of a Current Time value by the singlemodule, and the distribution to, and utilization by, multiple objectswithin a player of the same Current Time value has a desirable result ofkeeping all objects synchronized.

Typically the Current Time value must be regularly and accuratelyupdated by the player, or the presentation of media content will befaulty. For instance, if the Current Time value is not updatedsufficiently often, a video component of a media stream may appearuneven or jumpy, and gaps in an audio content may be audible.

Although the concept of Current Time seems straightforward, in fact, itconflates two subtly different properties of media playback. The firstproperty of media playback that is conflated in the concept of CurrentTime is a time elapsed since the beginning of the media contentpresentation (hereafter called “Presentation Time”). Thus, if the mediahas been playing for one minute, the value of Presentation Time is60,000 milliseconds. All of time values discussed herein can be measuredin various units. Two popular units are milliseconds, andcenti-nanoseconds, or {fraction (1/10,000,000)} of a second. The unit ofmeasurement is not an issue here. Other considerations of representingtime that are not issues here are the precision, the range of values,and the format of the representation.

The second property of media playback that is conflated in the conceptof Current Time is a location in the media content stream that iscurrently being played (hereafter called “Content Time”). In atraditional linear media stream that is always played back at a fixed,“normal” rate, any given content element is always presented after afixed amount of time has elapsed from the beginning of playback. Becauseof this, each such content element can be regarded as having a timestampassociated with it, i.e., a time value specifying how long it would taketo reach that location, starting from the beginning of the mediacontent, and playing at normal rate. Hereinafter we will call thisproperty “Data Time.”

Presentation Time and Data Time are identical in traditional players,because traditional players can only present media content at a fixed“normal” rate. However, when a player is enhanced with a Time-ScaleModification (TSM) capability, it can present media content at variousrates. Because of this, Presentation Time and Data Time are no longerthe same. For example, if a 60-second clip of media content is presentedat a fixed rate that is twice normal rate, at the end of the clip theData Time is 60,000 milliseconds, but the Presentation Time is 30,000milliseconds. This is because it only takes 30 seconds to play the60-second clip.

We have discovered that problems may occur when a traditional player isenhanced with TSM functionality. In particular, if a Current Time valueis distributed to multiple objects, some of them may interpret theCurrent Time value as specifying Data Time, some of them may interpretthe Current Time value as specifying Presentation Time, and some of themmay interpret the Current Time value as specifying both Data andPresentation Time. Thus, a first problem occurring when a traditionalplayer is enhanced with TSM functionality is that the significance ofthe time value distributed to multiple objects is, in general,ambiguous. A second problem occurring when a traditional player isenhanced with TSM functionality is that Data Time does not, in general,equal Presentation Time, and the calculation, storage, and distributionof a single time value is inadequate to specify both values.

It is quite common for media players to rely on an audio renderer (forexample, a player object responsible for outputting audio contentthrough, for example, a computer sound card) to calculate and update theCurrent Time value. This is done because the nature of audiorepresentation is such that typically each audio data element has eithera fixed, or explicitly specified presentation duration, associated withit, and these presentation durations are enforced by audio renderinghardware. Therefore, the audio renderer can typically determinePresentation Time either by maintaining a running total of thepresentation durations of all audio data elements rendered sinceplayback began, or in some cases by querying the audio renderinghardware itself for the equivalent value.

If a media player does in fact acquire the Current Time value from theaudio renderer, the value that the audio renderer will return to thesystem will typically be the Presentation Time. Since most of the restof the system needs Data Time, most of the rest of the system can nolonger employ the value returned by the audio renderer object.

As one can readily appreciate from the above, a need exists in the artfor a method and apparatus for solving one or more of theabove-described problems.

SUMMARY OF THE INVENTION

One or more embodiments of the present invention advantageously satisfyone or more of the above-described problems. In particular, one or moreembodiments of the present invention provide a method for managingPresentation Time in a digital rendering system for presentation oftemporally-ordered data when the digital rendering system includes aVariable Rate Presentation capability. Specifically, one embodiment ofthe present invention is a method for converting Presentation Time toData Time, and for reporting Data Time instead of Presentation Time whenonly one time can be returned.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 shows a block diagram of a Presentation System embodied as aRealNetworks® RealPlayer® application running on a computer; and

FIG. 2 shows a block diagram of a generalized Presentation system thatincludes Presentation System Controller Modules, Other PresentationModules (including a Presentation Rate Modification Module), and anumber of Renderers.

DETAILED DESCRIPTION

In accordance with one embodiment of the present invention, PresentationSystem 100 (a more general definition of a Presentation System isprovided below) is embodied as a RealNetworks® RealPlayer® applicationrunning on a computer, for example, under some version of the MicrosoftWindows operating system. As shown in FIG. 1, application module 110sends to, and receives from, Player Core object 120, control and statusmessages such as, for example, Play, Pause, Stop, and so forth. TemporalSequence Presentation Data, also referred to herein as PresentationData, (a more general definition of Temporal Sequence Presentation Datais provided below) is embodied as streaming media content and isdelivered to the RealPlayer® application over the Internet, a local-areanetwork (LAN), or from files stored in the computer that is executingthe RealPlayer® application. For example, in accordance with oneembodiment, the Presentation Data, for example, audio data, is receivedby media content source module(s) 130, and are placed in audio mediadata buffers 140 that are made available to Player Core object 120.

As will be defined in more detail below, each data element of thePresentation Data has a Rendition Type that corresponds to a type ofRenderer (a more general definition of Renderer is provided below) thatcan be used to render the data element. For example, for the embodimentdescribed above, the Rendition Types that can be rendered include butare not limited to audio (encoded in various formats), video, stillimages, text, and scripts. In particular, audio is a Time-DistinguishedRendition Type (a more general definition of Time-DistinguishedRendition Type is provided below). As a result, for this embodiment,audio is organized within the RealPlayer® application in buffers thatcontain, for example, 100-milliseconds of sample values. Further, everybuffer is timestamped, so these buffers are Timestamped Elements (asmore generally described below, this means that the Data Time of theelement is explicitly represented as part of the element), and the timeassociated with the first sample in every buffer is specified inmilliseconds.

In accordance with this embodiment, the Rendition Period (as moregenerally described below, this is the length of time the renderingprocess should last for the element) of the audio buffers is 100milliseconds. In some ways the individual audio samples play the part ofthe data elements as described below. It would be obvious to someone ofordinary skill in the art how to sometimes regard 100 millisecondbuffers of samples and sometimes the individual samples themselves asaudio elements. In accordance with this embodiment, a sample period ofthe individual audio samples is stored in a header that is part of thesample buffer definition (for example, one 10,000^(th) of a second is atypical sample period).

In accordance with this embodiment, an object called TSMAudioDeviceobject 150 combines functions of the Renderer for audio data(TSMAudioDevice Audio Renderer 160) and a Variable Rate PresentationModule (a more general definition of Renderer is provided below)(TSMAudioDevice VRP Module 170). In accordance with this embodiment, theaudio Renderer part of TSMAudioDevice object 150 (i.e., TSMAudioDeviceAudio Renderer 160) is a Timing Renderer (a more general definition of aTiming Renderer is provided below) for Presentation System 100. Notethat the RealNetworks® RealPlayer® application does not include supportfor variable rate playback. However, Plug-In 180 comprises a productcalled a 2xAV Plug-In is available from Enounce, Incorporated of PaloAlto, Calif. When the 2xAV Plug-In is installed on a computer that hashad the RealPlayer® application previously installed, it “plugs into”the RealPlayer® application, and adds variable rate playback capabilitythereto. The 2xAV Plug-In has its own User Interface, which includes aslider that a user can manipulate to adjust playback rate. In operation,the 2xAV Plug-In communicates with TSMAudioDevice object 150 by sendingmessages through an object called State Information Exchange Server 190(“SIX Server 190”).

Thus, in accordance with this embodiment, TSMAudioDevice object 150accepts messages from SIX Server 190 that specify a desired playback orpresentation rate. Playback or presentation rate values can range from0.3 to 3.0 (a rate of 1.0 is normal; a rate of 0.3 is 30% of the normalrate; and a rate of 3.0 is three times faster than the normal speed).TSMAudioDevice object 150 receives SIXExchange messages from SIX Server190, and stores a requested playback rate value contained in thesemessages as a new value of an internal Current Presentation Rateparameter or property. In addition, as shown in FIG. 1, TSMAudioDeviceobject 150 receives buffers 200 of audio data to be rendered (i.e.,played out through the computer's sound card) from Player Core object120. When TSMAudioDevice object 150 receives buffers 200 of audio datato be rendered, it is processed by TSMAudioDevice VRP Module 170.TSMAudioDevice VRP Module 170 processes buffers 200 through a library ofsignal processing routines, for example, a suitable library of signalprocessing routines called the Time Scale Tailor package is availablefrom Enounce, Incorporated of Palo Alto, Calif. In accordance with thisembodiment, this library carries out digital signal processingprocedures on buffers 200 of audio samples that has the effect ofreducing the number of samples in the buffer (when playing faster thanreal time) or increasing the number of samples in the buffer (whenplaying slower than real time), thereby effectively changing theplayback rate. For example, in accordance with this embodiment,processing the buffer using the library decreases or increases thesamples in a particular way so as to leave the perceptual and linguisticinformation in the buffers unchanged, but to change the duration of thebuffers. Additionally, playback rate parameters, unmodified and modifiedbuffer lengths and Rendering Period values, and other time-relatedvalues are calculated by TSMAudioDevice VRP Module 170, and are storedwith each audio buffer. Then, modified audio data buffers 210 are storedby TSMAudioDevice VRP Module 170 for presentation by TSM AudioDeviceAudio Renderer 160. TSM AudioDevice Audio Renderer 160 comprises AudioRenderer Core Object 165 that submits modified buffers 210 forprocessing to the computer's audio electronics, for example, ComputerSound Card 220. For example, as shown in FIG. 1, Core Object 165comprises an interface known as a WAV driver. This interface is definedby Microsoft, and is supported by the Windows operating system.

On a regular basis during playback or presentation, Player Core object120 calls a method implemented by TSMAudioDevice object 150 calledGetCurrentAudioTime( ). This method returns a Current Time inmilliseconds. Additionally, every time a buffer of audio samples isprocessed (for example, buffer 200), TSMAudioDevice object 150 isresponsible for calling a Player Core object 120 method calledOnTimeSync( ), and passing to the Player Core object 120 method theCurrent Time in milliseconds. Player Core object 120 interprets both ofthese times as Data Times. In this embodiment, Presentation System 100(other than TSMAudioDevice object 150) does not support the concept ofPresentation Times that are different than Data Times. To do this, inaccordance with one embodiment of the present invention, TSMAudioDeviceobject 150 carries out methods described below to convert PresentationTime (for example, as reported by its WAV driver Core object routines)into Data Time (as needed by Player Core object 120).

Before describing the methods to convert Presentation Time into DataTime, we present generalizations of the matters described above inconjunction with FIG. 2. In particular, FIG. 2 shows a block diagram ofa generalized Presentation system that includes: Presentation SystemController Modules 400, Other Presentation Modules 410 (including aPresentation Rate Modification Module), and a number of Renderers, forexample, Audio Renderer 420, Video Renderer 430, and Other Renderers440. Further, as shown in FIG. 2, Temporal Sequence Presentation Data450 is applied as input to Other Presentation Modules 410.

As defined herein, a Presentation System means a system or method that:(a) acquires, interprets, decodes, and manages presentation of TemporalSequence Presentation Data (defined below); (b) selects, instantiates,initializes, controls, and monitors Renderers (defined below); (c)initiates a presentation by determining which presentation data elementsare to be submitted first to which Renderers, effecting such submission,and causing the Renderers to begin processing; (d) maintains a CurrentTime parameter, whose value is regularly updated in amonotonically-increasing fashion during linear presentation (the valuemay be set to zero or any other value when presentation is stopped, or ajump is performed to a non-sequential location)—the Presentation Systemmay maintain and update the value of the Current Time parameter byidentifying a Renderer that can reliably maintain and update itsCumulative Rendition Period, and arrange to acquire the value of thatparameter at regular intervals; (e) distributes its Current Timeparameter to other Presentation Modules as needed; and (f) manages thepresentation process, including by determining which data elementsshould be submitted next to which Renderers, and by comparing itsCurrent Time value to the Data Time of those data elements, therebydetermining when to effect such submission.

A digital media player is a common type of Presentation System, butthere are many other types of Presentation Systems. For example, acontroller that processes a script which causes digitally-controlledmanufacturing equipment to make a printed circuit board is also aPresentation System, as is a controller that processes a script whichcauses a robot to perform a dance.

As defined herein, a Renderer is a system or method having the followingcharacteristics: (a) the Renderer processes Temporal SequencePresentation Data (defined below); (b) the Renderer processes dataelements in an ordered sequence in which “earlier” elements areprocessed before “later” elements (the order may be determined by theorder in which the elements are submitted to the Renderer, or by theData Times (defined below) of the elements, or by using othertechniques); (c) processing a data element takes a finite amount of time(possibly but not typically zero) known as the Rendition Period of thedata element; (d) processing a sequence of data elements takes a finiteamount of time directly related to the sum of the Rendition Periods ofthe individual elements, and, potentially, some other factors (theamount of time required to process (render) a sequence of data elementsis called a Cumulative Rendition Period for those elements); and (e) atleast one instance of a Renderer (often associated with rendering ofaudio data) has a capability of reporting back to a module, for example,a Presentation System Control Module, upon request, a current value ofthe Cumulative Rendition Period (a Renderer that is consistently used bythe Presentation System in this fashion is referred to as a TimingRenderer).

As defined herein, Temporal Sequence Presentation Data, also referred toas Presentation Data, means data having the following characteristics:(a) the purpose, utility, or semantics of the data is closely associatedwith its presentation—presentation involves rendering of the data toachieve some effect (including but not limited to constituting a visibleand/or audible presentation that can be monitored by a human being); (b)there are a plurality of rendering processes capable of effecting anappropriate presentation of the data; (c) the data comprises a set ofelements; (d) each data element has a Rendition Type that corresponds toa type of Renderer that can be used to render the data element—somecommon Rendition Types are Pulse Code Modulation (PCM) audio, MPEGvideo, and JPEG images; (e) one or more Rendition Types may beTime-Distinguished Rendition Types—Time-Distinguished Rendition Typesare Rendition Types of Temporal Sequence Presentation Data whoseintrinsic characteristics and whose natural rendition process make thempreferred candidates for defining and maintaining a system-wide CurrentTime parameter (note that most audio Rendition Types areTime-Distinguished Rendition Types); (f) associated with each element isa Data Time—the Data Time of some elements may be explicitly representedas part of the element (such elements are called Timestamped Elements),and the Data Time of some elements may be derivable only by performingor simulating an appropriate rendering process on all or part of thePresentation Data (such elements are called Sequential Elements); (g)the elements have a partial ordering, so that when performing renderingoperations on the data it is possible to determine i) which dataelements to deliver to the Renderers to begin the presentation process;and ii) given that the presentation process has reached a certain point,which data elements to deliver to the Renderers next to continue thepresentation process; and (h) associated with each element is aRendition Period—the Rendition Period is the length of time therendering process should last for that element, where the RenditionPeriod of an element may be specified in many different ways, includingbut not limited to the following: (i) as a value explicitly stored aspart of the element, (ii) as a fixed value associated with that type ofdata element, and stored in a header field of the Presentation Data,(iii) as a fixed value associated with a Presentation System, (iv) adifference between the Data Time of the element and the Data Time of afollowing element that would be submitted to the same Renderer in thecourse of presentation (i.e., the element is rendered until there isanother element to be rendered by the same Renderer), (v) as a fixedproperty of the rendering process.

As defined herein, a Variable Rate Presentation (“VRP”) Module (alsoknown as a VRP Module) means a module that: (a) accepts control commandsand messages from the Presentation System; (b) maintains and, inresponse to commands from the Presentation System, updates the value ofa Current Presentation Rate parameter where values of this parameterhave the following interpretation: (i) a value of 1.0 means thatpresentation is to take place at the “normal” or default rate, (ii) avalue less than 1.0 but greater than zero means that presentation is totake place slower than “normal” (the factor by which presentation is tobe slowed down is equal to the inverse of the Current Presentation Ratevalue), (iii) a value greater than 1.0 means that presentation is totake place faster than “normal” (the factor by which presentation is tobe sped up is equal to the Current Presentation Rate value), and (iv) avalue less than zero is interpreted identically to the correspondingpositive value of the parameter, but the direction of presentation isreversed (i.e., the presentation “runs in reverse”); (c) processesTemporal Sequence Presentation Data; (d) modifies Temporal SequencePresentation Data in a manner consistent with the value of the CurrentPresentation Rate parameter and the Renderers to which the data will belater submitted, having the effect that the rate with which processingtakes places will track the value of the Current Presentation Rateparameter. The implementation of Variable Rate Presentation may alsoinvolve one or more Renderers. In this case the value of the CurrentPresentation Rate parameter is attached to the data elements, orotherwise communicated to the appropriate Renderers.

In a Presentation System fabricated in accordance with one embodiment ofthe present invention, the Presentation System would maintain twoseparate values of the Current Time parameter. The first of the valueswould be a Current Data Time value that indicates the largest Data Timevalue of any data element that has already been submitted for rendering,or should have already been submitted for rendering. The second of thevalues would be a Current Presentation Time value, which would be theCumulative Rendition Period of all data elements submitted sincepresentation began (i.e., the elapsed rendering time). In a PresentationSystem fabricated in accordance with another embodiment of the presentinvention, the Presentation System would maintain a single value of theCurrent Time parameter. Such an embodiment is typical of PresentationSystems that were not designed with the notion of variable rate playbackin mind. More specifically, some such systems were designed with animplicit assumption that the only possible presentation rate was 1.0.

Presentation Time and Data Time are identical properties in traditionalPresentation Systems, such as media players. However, when aPresentation System is enhanced with a Variable Rate Presentation(“VRP”) capability, these properties are no longer the same. We havediscovered that this presents a problem when a traditional media playeror other Presentation System is enhanced with a VRP capability, for tworeasons. First, if a Presentation System Control Module only acquires asingle Current Time value from a Timing Renderer, it is probablyassuming that that value can be interpreted as both Current Data Timeand Current Presentation Time. If these times are not equal, at leastone of those assumptions will be in error. Secondly, if a single CurrentTime value is distributed to multiple components, some of whichinterpret the value as Current Data Time, and some of which interpretthe value as Current Presentation Time, at least one of theseinterpretations will be in error. We have invented a way to convertPresentation Time to Data Time, and we have invented a method forreporting Presentation Time when only one time can be returned.

In accordance with one embodiment of the present invention, certaintime-related properties (that will later be used to calculate CurrentPresentation Time and Current Data Time) are associated with TemporalSequence Presentation Data elements by taking the following steps.

Step 1: from time to time as a user or some other controlling entitydecides to change a rate of presentation, the Presentation SystemControl Module sends a message to a Variable Rate Presentation Module(VRP Module), specifying an updated value for a Current PresentationRate parameter. When the VRP Module receives this message, it updatesthe value of its Current Presentation Rate parameter.

Step 2: in preparation for presentation, the Presentation Systemorganizes Temporal Sequence Presentation Data elements (for example,audio samples) into collections called buffers.

Step 3: buffers are presented in an ordered or semi-order fashion forPresentation Rate modification and rendering, typically according to theData Time of the first data element in a buffer.

Step 4: the number of unmodified samples in each buffer is determined,and the Unmodified Rendition Period of each element is obtained. Thevalue of the Current Presentation Rate parameter is held constant (i.e.,it is not allowed to change) while the VRP Module is processing abuffer. Also, the Rendition Period of all data elements in a buffer isconstrained to be equal. Note, however, if it were desired to varyeither the Current Presentation Rate or the Rendition Period within abuffer, that buffer could be broken down into multiple smaller buffersin which those properties were constant. In doing so, if necessary, eachbuffer could hold only a single data element.

Step 5: the Unmodified Cumulative Rendition Period for the buffer iscalculated and retained as a property of the buffer by multiplying theRendition Period of each data element in the buffer by the number ofunmodified data elements in the buffer.

Step 6: the Data Time of the buffer is defined to be the Data Timeassociated with the first unmodified data element in the current buffer.The Data Time is acquired or calculated, and retained as a property ofthe buffer. If it is not directly specified as a property of the firstdata element of the current buffer, it can be calculated by adding theData Time of the previous buffer to the Unmodified Cumulative RenditionPeriod of the previous buffer.

Step 7: the data elements in the current buffer are presentation ratemodified, so that the ratio of the Cumulative Rendition Period of thebuffer prior to presentation rate modification, divided by theCumulative Rendition Period of the buffer following modification, issubstantially equal to the Current Presentation Rate.

Step 8: the number of modified data elements in the modified buffer, andthe Modified Rendition Period of each data element in the buffer, isdetermined and retained as a property of the buffer.

Step 9: the Modified Cumulative Rendition Period for the buffer iscalculated and retained as a property of the buffer by multiplying theModified Rendition Period of each data element in the buffer by thenumber of modified data elements in the buffer.

Step 10: the Modified Presentation Time of the buffer is defined to bethe Presentation Time associated with the first modified data element inthe buffer. This time is calculated and retained as a property of thebuffer. It is calculated by adding to the Modified Presentation Time ofthe first modified data element of the previous buffer, the ModifiedCumulative Rendition Time of the previous buffer.

Step 11: calculate, and retain as a property of the current buffer, theCumulative Modified Data Element Count associated with the first dataelement in the current buffer by adding the Cumulative Modified DataElement Count associated with the first modified data element in theprevious buffer to the number of modified data elements in the previousbuffer.

Note that in this embodiment only the VRP Module needs to be informed ofthe current value of the Presentation Rate parameter. Renderer Modules,on the other hand, get all of their information about Presentation Ratefrom the buffer properties described above.

In accordance with one embodiment of the present invention, eachRenderer is assumed to comprise a Core component. This Core componentmay be hardware, software, or a combination of hardware and software.For example, the Core component of an audio Renderer may be a Sound Cardand its associated driver software. The Core component performs theessential rendering process for the particular Type of Temporal SequencePresentation Data that the Renderer processes.

In accordance with one embodiment of the present invention, the Corecomponent can be asked at any point in time to report the number of dataelements rendered since a distinguished event such as, for example, aninvocation of an Initialize or Clear command. Equivalently, the Corecomponent rendering hardware or software may be able to report thenumber of milliseconds of rendering that has occurred since adistinguished event.

Renderers, especially Timing Renderers, must decide how to respond whenother components of the Presentation System ask for the Current Timevalue without specifying whether Presentation Time or Data Time isdesired. In many cases it is possible to determine that the PresentationSystem really wants to know what the Current Data Time is. Therefore inaccordance with one embodiment of the present invention, a certain DataTime is returned when a request is made for the Current Time. For this,and other reasons, Renderers, especially Timing Renderers, must maintainan up-to-date and accurate value for both the Presentation Time and theData Time associated with the data element currently being rendered. Inaccordance with one embodiment of the present invention, it is the DataTime of the data element currently being rendered by the Core componentthat is returned as the Current Time when the Current Time is requestedby another module. Therefore, in accordance with one embodiment of thepresent invention, Current Presentation Time and Current Data Time arecalculated by taking the following steps.

Step 1: a modified buffer with its associated properties as describedabove is submitted to an appropriate Renderer.

Step 2: if the Renderer's Core component is capable of reporting thenumber of milliseconds of rendering that has occurred since adistinguished event, and the Modified Presentation Time of this modifiedbuffer is zero (or some other distinguished value), the Renderertriggers the distinguished event in its Core component.

Step 3: the Renderer submits the contents of the buffer to its Corecomponent.

Step 4: the Renderer also stores each modified buffer in some methodthat allows ready access to all of the buffer properties until it hasdetermined that all data elements in the buffer have been rendered.

Step 5: if the Core component is capable of reporting the number of dataelements rendered since a distinguished event occurred, the Renderercalculates the Current Data Time and the Current Presentation Time usingthe following algorithm.

Step 5a: it asks its Core component for the cumulative number of dataelements rendered.

Step 5b: it determines which buffer the next data element to be renderedwill have come from, by identifying the particular buffer that has forits Cumulative Modified Data Element Count the largest cumulative samplenumber less than or equal to the reported number of data elementsrendered—this buffer is referred to as the current rendering buffer.

Step 5c: the current Data Time is equal to the Data Time of the currentrendering buffer, plus an offset.

Step 5d: the offset is calculated by multiplying the unmodified bufferduration by the Completion Fraction.

Step 5e: the Completion Fraction is calculated by subtracting thecumulative sample number associated with the first sample in the currentrendering buffer from the Core component's reported number, and dividingthe result by the number of modified samples in the buffer.

Step 5f: the Current Presentation Time is equal to the ModifiedPresentation Time of the current rendering buffer, plus an offset.

Step 5g: the offset is calculated by multiplying the buffer's ModifiedCumulative Rendition Period by the Completion Fraction.

Step 6: if the Core is capable of reporting the number of millisecondsof rendering that has occurred since a distinguished event, the Renderercalculates the Current Data Time and the Current Presentation Time usingthe following algorithm.

Step 6a: it asks its Core component for the number of milliseconds ofrendering that has occurred.

Step 6b: it determines which buffer the next data element to be renderedwill have come from by identifying the particular audio buffer that hasfor its Modified Presentation Time the largest value less than or equalto the Core's reported value—this buffer is referred to as the currentrendering buffer.

Step 6c: the current Data Time is equal to the Data Time of the currentrendering buffer, plus an offset.

Step 6d: the offset is calculated by multiplying the unmodified bufferduration by the Completion Fraction.

Step 6e: the Completion Fraction is calculated by subtracting theModified Presentation Time of the current rendering buffer from the Corecomponent's reported value, and dividing the result by the CumulativeModified Rendering Period of the buffer.

Step 6f: the Current Presentation Time is equal to the ModifiedPresentation Time of the current rendering buffer, plus an offset.

Step 6g: the offset is calculated by subtracting the ModifiedPresentation Time of the current rendering buffer from the Corecomponent's reported value.

Step 6h: the current Data Time is reported to the player as thePresentation Time.

Step 7: whenever an object requests the Current Time, the Renderercomputes an updated value for the Presentation Time and Data Time, andreports either or both times as appropriate.

Those skilled in the art will recognize that the foregoing descriptionhas been presented for the sake of illustration and description only. Assuch, it is not intended to be exhaustive or to limit the invention tothe precise form disclosed.

What is claimed is:
 1. A method for associating time related propertieswith Temporal Sequence Presentation Data elements in a digital renderingsystem having a Variable Rate Presentation capability, the methodcomprising steps of: receiving a rate of presentation, and updating aCurrent Presentation Rate parameter; organizing the Temporal SequencePresentation Data elements into buffers; presenting the buffers forpresentation rate modification and rendering; determining a number ofunmodified data elements in each buffer and an Unmodified RenditionPeriod of each data element, and retaining, as a property of eachbuffer, an Unmodified Cumulative Rendition Period for the buffer;determining and retaining as a property of the buffer, a Data Time;presentation rate modifying the data elements in the buffer and storingthem in a modified buffer; determining and retaining, as a property of amodified buffer, a number of modified data elements in the modifiedbuffer and a Modified Rendition Period of each data element in themodified buffer; determining and retaining, as a property of themodified buffer, a Modified Cumulative Rendition Period for the modifiedbuffer; determining and retaining, as a property of the modified buffer,a Modified Presentation Time of the modified buffer; and determining andretaining, as a property of the modified buffer, a Cumulative ModifiedData Element Count associated with a first data element in the modifiedbuffer.